ARCHIVE: Australian Ham Radio Discussion Forum ( AHRDF )

Hello,

At my new QTH the opportunities are a bit limited for antenna towers so I am thinking to install a tilt-up home brew mast of 6m height in the block corner.

My question relates to the better material profile to use - circular hollow section, or square hollow section.   The material will be aluminium for weight considerations.   I can arrange a nicely-anchored stirrup structure in concrete for the base, but being in a corner I cannot deploy guy wires on three or four sides so it must be effectively self supporting.   At a pinch I could arrange for stays, also in aluminium, on two of the sides that are within my property line.   There is the need for it to appear "aesthetically pleasing" to avoid conflict with ZYL and harmonics.

So, which is the better section type to use for stability?   In my area there is little to worry about regards wind velocity and I do not intend to have anything really "top heavy" on the mast.   Indeed log periodics and yagis are not going to be possible due to their overhang of neighbouring properties.

I hope that I have made myself clear?   Alternative suggestions welcome of course, if my approach is too "brute force."  

73
Denis VK6AKR

Hey Denis,

Have a look at NBS Antennas for their tilt-over mast.  Perhaps build something similar.
https://www.nbsantennas.com.au/masts-products.html

I normally wouldn't touch this with a "ten foot pole", either round or square section, but since you haven't had a lot of responses, I will contribute a few thoughts.

Aluminium : tensile strength is the main criteria. As you elevate the tube from horizontal and particularly if you already have a head load attached to the top, it will bend - and as I have experienced previously - that happens most at about 40-50 degrees elevation. The issue is whether that will be a permanent or temporary bend.

I use 'scaffolding tube' for the pipes out of the top of the rotators here. It is rated T6061 and has dimensions 44.44 mm OD x 4.47 mm wall thickness.  It comes in 6.1 metre lengths and weighs 1.67 Kg per metre.  That would probably be the best tensile strength readily available - and is fairly cheap too.

You could use it as the flagpole-style base as well as the pivoting arm. If you plan to use a rotator on top, buy an extra length plus some thick aluminium checkerplate to make the rotator support L-bracket and a second L-bracket for the top thrust bearing support. Clamp them on to the scaffold tube using 50mm muffler clamps allowing more than a metre between the two L-brackets. The further down the tube the rotator weight is, the lower the top load so if you used almost the whole 6.1M as a rotator tube, that places the load right towards the bottom as well as giving you virtually a double-strength tube during the lift.

Now about that temporary/permanent bend. You can use a permanent gin pole off the upper side of the tube to take the pressure off the tube as you elevate or you can do the cross-over spars that are used on yacht masts to also protect it when vertical. https://en.wikipedia.org/wiki/Spar_(sailing), bottom picture on https://en.wikipedia.org/wiki/Mast_(sailing)

The cross spars should probably be galv'd steel angle ( and for a single gin pole arm if used) as aluminium angle doesn't have the tensile strength.  So that requires two steel cross bars, slotted or drilled at each end, clamped around mid-way up the available tube plus a set of 4 stainless wires through the holes/slots, clamped near the top and bottom and then properly tensioned with turnbuckles. That provides the extra mechanical strength that the T6061 tube can't provide by itself during the lift process.

I suspect part of the lack of replies was that you didn't provide enough raw information in your post. Expected height, sizing, loading, antennas,....    Better to provide plenty of your initial thoughts and allow others to provide suggestions to alter them..

73 Doug

(05-04-2019, 07:37 AM)VK4ADC Wrote: [ -> ]I normally wouldn't touch this with a "ten foot pole", either round or square section, but since you haven't had a lot of responses, I will contribute a few thoughts.
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I am not qualified to comment on the engineering aspects but there are some other things that need to be considered before building.

What about council regulations?  In my local area building permits are not required for a mast less than 10m high but conditions vary from area to area. The council may require engineering drawings and assessment before giving permission. In that case the simplest solution would be to buy a commercial product which already has the engineering data. In some cases the council will also require consent from neighbors.

What about insurance?  Before erecting the antennas at my current location I sought advice from a friend who is a retired insurance assessor. He said that it was unlikely the an insurance company would insure a home-built mast, but the most important thing to consider is what happens if the mast or antenna falls and hits a neighbor's house, or more seriously the neighbor? It is very unlikely that the neighbor's insurance would cover that event and it could end up in a court. His advice was: " make sure that it will land on your own property if it does fall over." So I have limited the height of my mast to 8m to comply with that suggestion.

Another local ham who is a workplace and safety officer also told me that any winch used to raise a lower a mast must be by law, "self braking. Using a simple boat winch will negate any insurance claim for personal injury. Self-braking winches are really not that expensive anyway.

73

Wayne VK4WDM

(04-04-2019, 10:56 PM)VK6RK Wrote: [ -> ]Hey Denis,

Have a look at NBS Antennas for their tilt-over mast.  Perhaps build something similar.
https://www.nbsantennas.com.au/masts-products.html

Thanks Rick

Yes a brief review of their site is impressive.   I'll get in touch with them, thanks for the heads-up.

73
Denis VK6AKR

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(05-04-2019, 07:37 AM)VK4ADC Wrote: [ -> ]I normally wouldn't touch this with a "ten foot pole", either round or square section, but since you haven't had a lot of responses, I will contribute a few thoughts.
...

Thanks Doug

Excellent suggestions and I had quite ignored the spar systems as used on yachts.   There's clearly more to this than initially considered.

In terms of height I had not thought to go much beyond 6m due to the close proximity to boundary fences and the need to keep things as compact as possible.   Commercial masts as suggested in Rick and Wayne's posts might be far the better way to go.

73

Denis VK6AKR

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(05-04-2019, 10:53 AM)VK4WDM Wrote: [ -> ]I am not qualified to comment on the engineering aspects but there are some other things that need to be considered before building.

....

Thanks for the advice Wayne.

I see the wisdom of involving Council, engineers, etc and making sure that insurers are on side.   On balance it might be best to spend "some of the childrens' inheritance" on a properly engineered and installed system.   Then I can concentrate on the electronics and communication aspects of the hobby!

73
Denis VK6AKR

P.S. I need to look a bit more closely I think, at how I should be replying here to avoid taking too much "bandwidth" with others' embedded replies.   Sorry if I have transgressed thus far!

Edited to remove dup'd text - Admin

Hello Again Denis

If you are only going up 7m or so, there are no council requirements and the antenna and rotator are relatively light such as a VHF yagi, I don't see any reason why you can't make a simple tilt-over mast.

I have attached some photos of mine.  It is 8m high, made from 60mm ally tube with a wall thickness of 3.5mm. The lowest antenna is a Create CLP-5130N LP that covers from 50-1300 MHZ. Its a bit low on 23cm gain so I have a separate 18el yagi for that band. The self braking winch lowers and raises it with ease.

I have also attached a photo of my ground-mounted Hustler 6-BTV HF vertical - an excellent antenna.

73

Wayne VK4WDM